This invention relates to a resettable thermal cut-off fuse designed for use in an electrical appliance provided with a heat source and adapted so that it keeps the circuit in a closed state while the appliance remains under its normal temperature condition; when the amount of heat radiated by the appliance abnormally increases so much as to elevate the ambient temperature to a prescribed danger level, the fuse opens the circuit by accurately responding to the change of temperature; and when the ambient temperature falls and returns to the normal level again, the fuse is rendered manually resettable to close the circuit.
Generally, the thermal cut-off fuse fulfils its purpose by breaking its own circuit (which usually provides electrical continuity between a pair of lead wires) at a prescribed temperature. This is done by use of a thermal pellet which changes from a solid state to liquid state at the prescribed temperature. Good many fuses developed to date amply satisfy this function.
The conventional thermal cut-off fuses using such a thermal pellet are of disposable types. Once they serve the purpose of opening the circit upon sensing an abnormal elevation of temperature, they cannot be reset and must be wholly discarded as no longer useful. The well-known bimetal type fuse has one advantage over the thermal cut-off fuse in that it is resettable and, therefore, is more economical. In terms of accuracy of response to temperature change, however, the bimetal type fuse can hardly stand comparison with the thermal cut-off fuse using the thermal pellet which enjoys outstanding accuracy of temperature response. Further the bimetal type fuse is of a self-resetting type and, for this reason, cannot be used as a thermal cut-off fuse. For example, when trouble develops and abnormal heat buildup ensues in an electrical system, the bimetal snaps to open the circuit and prevents further aggravation of the trouble. When the increased heat is dispersed into the ambient air and the temperature of the bimetal returns to a safe level, the bimetal automatically snaps back to close the circuit, whether the trouble has been completely eliminated or not. Unlike the thermal cut-off fuse, therefore, the bimetal is not capable of precluding the continuance of a dangerous temperature condition. This is because the bimetal, by nature, fulfils the sole purpose of temperature control. In contrast, thermal cut-off fuses of simple constructions cannot be reset like the bimetal can. Further, because they are relatively expensive, they are mostly produced in constructions which are not suitable for reuse. If they are manufactured in constructions which permit reuse, these constructions are preferably such that resetting can be done manually after it has been confirmed that the cause of the trouble has been eliminated.
An object of the present invention is to provide a thermal cut-off fuse of a simple construction using a thermal pellet having high accuracy of temperature response, which in use, when inserted in the circuit of an electrical appliance provided with a heat source, keeps the circuit in its closed state under the normal temperature condition, opens the circuit with a high accuracy of response when the ambient temperature reaches the prescribed danger level fixed in accordance with the ratings of the electrical appliance and can be manually reset to close the circuit when the ambient temperature falls and returns to the normal safe level.